Body Control Module 2001 GMC Sonoma

I've been told the BCM may be bad in my vehicle.

Drivers PW will not go down sometimes. Check engine light stays on as well as Air Bag light, ABS and Emergency Brake light come on and go off intermittantly. Interior lights will not come on when you you open the doors but if you use the keyless entry to unlock the door they do come on.

Where is the BCM located?
What steps are necessary to replace it?
Can you just install a used from a junkyard or does it need to be "FLASHED", whatever that means?